This invention relates to a joint for a large diameter casing adapted to extend deeply downwardly under the ground or water for use in the drilling or oil wells, and more particularly to a joint structure or configuration which provides excellent hermetically pressure tight capability and which is suitable for connection and coupling of large diameter casings.
Tubular members of large diameter have heretofore had their tube and joint sections subjected to an excessive tension stress resulting from their own weight when the tubular members are lowered to a predetermined depth within a well or the like. Loads of 200-300 tons at maximum are applied to the tubular member when suspended, and the tubular member is required to withstand pressure of 200-300 kg/cm.sup.2 per unit area in section. Thus, in the conventional joint, the connection with the tubular member must be designed to provide the maximum possible extent of thread contact. This however, may result in difficulty in connecting pipe sections. Alternatively, the joint may be constructed to allow for an easy and ready connection, but this however sacrifices sealing capability.
Successive connections of the tubular members impose an excessive torque on the joints as a result of tightening to the fullest possible extent to secure a pressure tight seal. Consequently, all of the joints may be deformed into a flat configuration and thus become unusable. Improvements on the external and internal thread configurations of such joints have been proposed to increase the yeild strength for a load on the tubular member when it is suspended. However, such proposals have failed, since the actual connecting operation leads to cross threading between the thread faces or turns. This results in cracks in the threaded portion crossing the axis of the tubular member and, contrary to reinforcing the yield strength, deteriorates the sealing capability of the joint. Accordingly, the casing is not usable over a long period of time.